Abstract:

The invention relates to a multileaf collimator having a plurality of
leaves mounted displaceably in an adjusting direction for establishing a
contour of a beam path. Each displaceably mounted leaf is assigned at
least one linear drive having at least one piezoelectric actuator for
displacing the leaf in the adjusting direction. Because the piezoelectric
actuator can be driven precisely, an improved radiation therapy can be
achieved, particularly in the case of a radiation therapy device having a
multileaf collimator of said kind, owing to precise establishing of the
contour.

Claims:

1.-10. (canceled)

11. A multileaf collimator, comprising:a leaf mounted displaceably in an
adjusting direction that establishes a contour of a beam path;a linear
drive assigned to the leaf that displaces the leaf in the adjusting
direction;a piezoelectric actuator arranged on the linear drive that
moves the linear drive; anda control device that drives the piezoelectric
actuator.

12. The multileaf collimator as claimed in claim 11, wherein the
piezoelectric actuator comprises a piezoelectric element and a transducer
coupled thereto.

13. The multileaf collimator as claimed in claim 12, wherein a frictional
force is transmitted between the transducer and the leaf as a driving
force and is adjustable as a function of a direction.

14. The multileaf collimator as claimed in claim 12, wherein the control
device drives the piezoelectric actuator to compliantly move the leaf by
the transducer during an excursion of the piezoelectric element in a
direction of motion and the transducer slides across the leaf in an
opposite direction of motion.

15. The multileaf collimator as claimed in claim 12, wherein the
transducer moves slower in a direction of motion than in an opposite
direction of motion.

16. The multileaf collimator as claimed in claim 11, wherein the
piezoelectric actuator is assigned to a narrow side or a flat side of the
leaf.

17. The multileaf collimator as claimed in claim 11, wherein the leaf is
moved by a plurality of piezoelectric actuators.

18. The multileaf collimator as claimed in claim 17, wherein the control
device successively drives the piezoelectric actuators.

19. The multileaf collimator as claimed in claim 17, wherein at least two
piezoelectric actuators are assigned to a narrow side or a flat side of
the leaf.

20. The multileaf collimator as claimed in claim 11, wherein the multileaf
collimator is used for a radiation therapy device.

21. A radiation therapy device, comprising:a retaining device; anda
multileaf collimator attached on the retaining device, wherein the
multileaf collimator comprises:a leaf mounted displaceably in an
adjusting direction that establishes a contour of a beam path,a linear
drive assigned to the leaf that displaces the leaf in the adjusting
direction,a piezoelectric actuator arranged on the linear drive that
moves the linear drive, anda control device that drives the piezoelectric
actuator.

22. The device as claimed in claim 21, wherein the piezoelectric actuator
comprises a piezoelectric element and a transducer coupled thereto.

23. The device as claimed in claim 22, wherein a frictional force is
transmitted between the transducer and the leaf as a driving force and is
adjustable as a function of a direction.

24. The device as claimed in claim 22, wherein the control device drives
the piezoelectric actuator to compliantly move the leaf by the transducer
during an excursion of the piezoelectric element in a direction of motion
and the transducer slides across the leaf in an opposite direction of
motion.

25. The device as claimed in claim 22, wherein the transducer moves slower
in a direction of motion than in an opposite direction of motion.

26. The device as claimed in claim 21, wherein the piezoelectric actuator
is assigned to a narrow side or a flat side of the leaf.

27. The device as claimed in claim 21, wherein the leaf is moved by a
plurality of piezoelectric actuators.

28. The device as claimed in claim 27, wherein the control device
successively drives the piezoelectric actuators.

29. The device as claimed in claim 27, wherein at least two piezoelectric
actuators are assigned to a narrow side or a flat side of the leaf.

[0002]The invention relates to a multileaf collimator, in particular for a
radiation therapy device, and to a radiation therapy device having a
multileaf collimator of said kind.

BACKGROUND OF THE INVENTION

[0003]A multileaf collimator is used in radiation therapy for treating
tumors. A multileaf collimator of such kind is described in, for
instance, DE 196 39 861 A1 and WO 00/46813. A tumor is irradiated during
radiation therapy with energy-rich beams, usually high-energy X-radiation
from a linear accelerator. The multileaf collimator is therein brought
into the path of the X-ray beam. The multileaf collimator has a plurality
of leaves that can be mutually displaced under motorized control for the
purpose of establishing an opening whose contour corresponds to that of
the tumor. Thus, only the tumor and not adjacent healthy body tissue will
be irradiated with the X-rays. Two sets of leaves are for that purpose
arranged mutually opposite such that they can be moved with their front
sides toward or away from each other. Virtually any tumor contour can be
reproduced in that way.

[0004]Said leaves can each be positioned by means of an electric motor
embodied as a stepping motor. The positioning accuracy of a stepping
motor of said type has, though, proved disadvantageous. Moreover, a
stepping motor has a starting behavior that does not allow slight
adjustment in positioning.

SUMMARY OF THE INVENTION

[0005]The object of the invention is to disclose a multileaf collimator
having an improved positioning device. Said object is inventively
achieved by means of a multileaf collimator as claimed in the claims.
Each leaf is for that purpose assigned at least one linear drive having
at least one piezoelectric actuator, which can be driven by a control
device, for displacing the leaf in an adjusting direction.

[0006]A high degree of positioning and repetition accuracy can be achieved
through employing a piezoelectric actuator for displacing a leaf. It is
hence possible to dispense with a complex measuring and control system
for compensating positioning inaccuracies of the kind needed even for
highly accurate stepping motors. Moreover, the displacement of the
piezoelectric actuator is proportional to the applied supply voltage. By
specifying the supply voltage it is thus possible to precisely and simply
specify how far the leaf will be displaced by means of the piezoelectric
actuator. A successive linear movement of the leaf can thus be achieved
by driving the piezoelectric actuator repeatedly.

[0007]The piezoelectric actuator furthermore consumes little current while
moving the leaf. The piezoelectric actuator is otherwise virtually
currentless so that its current consumption is close to zero.
Accordingly, a transformer requiring to be provided for electrically
powering the piezoelectric actuator can furthermore be of low-power
design. The energy requirements of the piezoelectric actuator are low so
that the operating costs of the linear drive are low compared with a
motor-driven linear drive. The noise produced while the transformer is
operating is low owing to the low power consumption.

[0008]A linear drive of said type can have very small dimensions because a
piezoelectric actuator occupies little structural space. It can therefore
be significantly more compact in structural design than a conventional
linear drive for a multileaf collimator having electric motors that can
be individually driven.

[0009]The piezoelectric actuator expediently has a piezoelectric element
and a transducer coupled to the piezoelectric element. The transducer can
in terms of its geometry in that way be matched exactly to the leaf
requiring to be moved.

[0010]In an advantageous development a frictional engagement is embodied
for transmitting a driving force between the transducer and the leaf
requiring to be moved, with the frictional force being adjustable as a
function of direction. In other words, when frictional contact has been
established with the transducer the displaceably mounted leaf is moved
thereby by means of static friction. The mechanical coupling between the
transducer and leaf due to frictional engagement is purely passive in
nature. There is hence no need to control the coupling force.

[0011]Because the transducer is linked directly to the leaf by means of
frictional engagement there is no mechanical play whatever between the
transducer and leaf. A particularly high degree of positioning and
repetition accuracy can hence be achieved. A complex control system for
compensating positioning inaccuracies of the kind needed even for highly
accurate stepping motors does not have to be employed.

[0012]Force is transmitted directly to the leaf requiring to be moved by
means of the transducer via frictional engagement. Gearing for force
transmission is hence not needed so that driving can be implemented
simply and economically. The transducer engages on the leaf virtually
without sound so that very little noise is produced while the leaf is
being moved. The current consumption of the piezoelectric actuator is
furthermore close to zero while the leaf is being held in a holding
position by means of frictional engagement so that a particularly low
energy consumption will be insured especially when the multileaf
collimator is in standby mode.

[0013]The control device expediently drives the piezoelectric actuator in
such a way that, exploiting the leaf's mass inertia, the leaf will be
moved compliantly during an excursion in the direction of motion and, in
the opposite direction, the transducer wilt slide across the leaf. In
other words the frictional engagement between the transducer and leaf is
produced solely by the interplay between the leaf's mass inertia and
direction-dependent driving of the transducer. Selective moving of the
leaf will have been achieved thereby in a simple manner and with little
control effort.

[0014]The control unit is advantageously set up for driving the
piezoelectric actuator in such a way that the transducer's speed will be
lower in the direction of motion than in the opposite direction. Rapid
buildup or cleardown of the supply voltage will cause the piezoelectric
element to expand or contract rapidly. The transducer secured to the
piezoelectric element will thus overcome the static frictional force
being applied to its friction surface through frictional engagement. The
transducer will be moved in the direction counter to the direction of
motion by means of sliding friction on the leaf's surface. The
transducer's contact point on the leaf can be changed in that way. What
is therein exploited is that the leaf requiring to be moved has a
significantly greater mass than the transducer and so will retain its
position owing to its mass inertia.

[0015]Slow buildup or cleardown of the supply voltage will cause the
piezoelectric element to expand or contract slowly. A frictional
engagement between the transducer and leaf will be produced in that way.
The leaf's mass inertia will be overcome by the static frictional force
between the transducer and leaf. The leaf will be displaced in the
direction of motion by means of the transducer engaging on it.

[0016]Thus, a linear leaf movement in the direction of motion can be
achieved in a simple manner with a periodic supply voltage that rises
rapidly and falls slowly. A reversal of the direction of motion can be
achieved just as simply with a periodic supply voltage that falls rapidly
and rises slowly.

[0017]A bilateral linear movement can accordingly be implemented using an
asymmetric supply voltage. For example a periodic voltage having the
nature of an asymmetric saw tooth is suitable as the supply voltage.

[0018]In an advantageous development, a plurality of piezoelectric
actuators are provided for moving the leaf. Thus, a leaf having a high
mass and hence a high mass inertia can also be moved by means of the
linear drive. The piezoelectric actuators can be moved jointly by means
of the control device so that the static frictional force transmitted by
means of static friction to the leaf will suffice to displace it by means
of frictional engagement. Moving of a leaf by means of a plurality of
simultaneously moved piezoelectric actuators is particularly significant
if a leaf having a high mass is to be moved. That is the case with, for
example, a multileaf collimator employed in radiation therapy. A
multileaf collimator of said type has leaves made of a
radiation-shielding material, usually a tungsten alloy, of very high
density so that the individual leaves have a high mass.

[0019]The control device is advantageously set up for operating the
piezoelectric actuators in succession. That will enable the individual
transducers' contact points to be changed successively by means of
sliding friction exploiting the leaf's mass inertia and allow the leaf to
be displaced continuously without interruption.

[0020]The piezoelectric actuators are therein expediently arranged on the
leaf's narrow and/or flat sides. Different advantageous arrangements are
therein possible. Thus a plurality of piezoelectric actuators can be
arranged in each case in pairs on opposite narrow sides or opposite flat
sides. The force applied to a linear guide holding and guiding the leaf
can in that way be reduced.

[0021]In another advantageous variant a plurality of piezoelectric
actuators can be arranged on a narrow or flat side. Structural space for
the piezoelectric actuator will then have to be provided only on said
narrow or flat side. The linear drive can then be of particularly compact
design. The leafs linear guide will on the other hand have to be embodied
in such a way that the leaf can be displaced smoothly notwithstanding the
force being applied unilaterally thereto.

[0022]The object is further achieved by means of a radiation therapy
device having a multileaf collimator as claimed in one of the preceding
claims. The claims directed to the multileaf collimator along with their
advantages are therein applicable analogously to the radiation therapy
device. Since the multileaf collimator has very precisely positionable
leaves, a contour for the irradiating of a tumor can be specified
precisely. That will allow radiation therapy to be performed with high
precision. The risk of either not including parts of the tumor tissue
during irradiating or of damaging healthy body tissue through irradiating
is therefore significantly less compared with a motor-driven multileaf
collimator according to the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]An exemplary embodiment of the invention is explained in more detail
below with reference to a drawing, in which:

[0030]FIG. 1 is a schematic top view of a multileaf collimator 2 that
includes a number of plate-type leaves 4 arranged substantially mutually
parallel. Said leaves 4 can be adjusted in the adjusting direction 6. For
adjusting, in each case two mutually opposite leaves 4 are with their
front sides 10 moved toward or away from each other by means of a control
device 8. It is in that way possible to set virtually any contour 12 for
the irradiating of a tumor by means of an X-ray beam traversing the
multileaf collimator 2 in the beam direction 14. In FIG. 1, as viewed
from the plane of the figure said X-ray beam traverses the irradiating
contour 10 from top to bottom through the multileaf collimator 2.

[0031]FIG. 2A shows a leaf 4 mounted longitudinally displaceably in the
adjusting direction 6 by means of a linear guide 16. For displacing the
leaf 4, a linear drive 18 for displacing a leaf 4 is provided with two
piezoelectric actuators 20,20' that can be driven by the control unit 8.
Bach piezoelectric actuator 20,20' includes a piezoelectric element
22,22' and a transducer 24,24' coupled thereto that are shown
schematically in FIG. 2A.

[0032]Both transducers 24,24' of the piezoelectric actuators 18,18' are in
frictional contact with the opposite narrow sides 26,26' of the leaf 4. A
frictional force 28,28' therein acts on the surface of the narrow side
26,26'.

[0033]The leaf 4 is displaced in a direction of motion 30 as follows. The
supply voltage V of the piezoelectric element 22 of the first
piezoelectric actuator 20 is first rapidly increased by means of the
control device 8. The transducer 24 of the piezoelectric actuator 20
slides by means of sliding friction across the surface of the narrow side
26. It therein covers the travel interval 34 in the direction 32 counter
to the direction of motion 30 and thus changes its contact point. The
supply voltage of the piezoelectric element 22' of the piezoelectric
actuator 20' is then rapidly increased by means of the control device 8
shown in FIG. 2B. The transducer 24' of the piezoelectric actuator 20'
will thus also be displaced in the opposite direction 32 on the surface
of the narrow side 26' by the extent of the travel interval 34. The
transducers 24,24' of both piezoelectric actuators 20,20' will then both
have a new contact point displaced in the opposite direction 32 by the
extent of the travel interval 34.

[0034]Finally, according to FIG. 2c, the supply voltages of the two
piezoelectric elements 22,22' are slowly simultaneously reduced by means
of the control device 8. The transducers 24,24' are both moved by the
piezoelectric elements 22,22' in the direction of motion 30 by the extent
of the travel interval 34. Thus the frictional forces 28,28' of both
friction surfaces of the transducers 24,24' will engage jointly via
frictional engagement on both narrow sides 26,26'. Since both transducers
24,24' are, moreover, moved slowly, the leaf 4 will by means of
frictional engagement also be moved compliantly with the transducers
24,24' in the direction of motion 30 by the extent of the travel interval
34. The contact point of both transducers 24,24' is then in turn changed
again as described for FIGS. 4a and 4b. A continuous linear movement of
the leaf 4 in the direction of motion 30 will have been achieved thereby.

[0035]Described in FIGS. 3a-c is the movement of the leaf 4 by means of
the linear drive 1 in the direction of motion 30 counter to the direction
of motion shown in FIGS. 2a-c. According to FIG. 3A, the supply voltage
of the piezoelectric element 22' is first rapidly reduced by means of the
control device 8. The transducer 24' of the piezoelectric actuator 20'
moves by the extent of the travel interval 34 in the direction 32 counter
to the direction of motion 30. According to FIG. 3B, the supply voltage
of the piezoelectric element 22 of the first piezoelectric actuator 20 is
then reduced by means of the control device 8. The transducer 24 of the
piezoelectric actuator 20 thus also moves by means of sliding friction by
the extent of the travel interval 34 in the opposite direction 32. The
contact point of both transducers 24,24' of both piezoelectric actuators
20,20' will in each case have been changed by the extent of the travel
interval 34,

[0036]Finally, according to FIG. 3c, the supply voltages of the two
piezoelectric actuators 22,22' are slowly simultaneously increased by
means of the control device 8. The transducers 24,24' are both displaced
in the direction of motion 32 by the extent of the travel interval 34.
Because said displacement takes place slowly and, moreover, both friction
surfaces of both transducers 24,24' transmit a frictional force 28,28' to
the leaf 4 by means of frictional engagement, the leaf 4 will likewise be
moved by means of static friction in the direction of motion 30 by the
extent of the travel interval 34.

[0037]Because both piezoelectric actuators 20,20' engage with their
transducers 24,24' on both opposite narrow sides 26,26' of the leaf 4,
the leaf 4 will not be subjected to any additional force. The linear
guide 16 can hence be embodied in a simple manner.

[0038]The movement, described in FIGS. 2a-c and FIGS. 3a-c, of the leaf 4
is intended solely to elucidate an interaction among a plurality of
piezoelectric actuators 20,20'. Leaves 4 having a very large mass can
basically be moved as a result of employing a larger number of
piezoelectric actuators 20,20'. Moreover, the piezoelectric actuators
20,20' will in that case not have to be individually driven consecutively
but can also be driven in groups. If the leaf 4 has a very high mass,
then owing to its mass inertia individual transducers 24,24' will also be
displaceable if driven relatively slowly. The number of piezoelectric
actuators 20,20', the driving thereof, and their friction surfaces will
hence be mutually coordinated to obtain an even movement.

[0039]According to FIG. 4, a second linear drive 18 for a leaf 4 has two
piezoelectric actuators 20,20'. The opposite flat sides 36,36' of the
leaf 4 are each assigned a piezoelectric actuator 20,20'. Each
piezoelectric actuator 20,20' exerts a frictional force 28,28' on the
flat side 36,36' assigned to it. The only difference compared with FIG.
2a is that both piezoelectric actuators 20,20' are now assigned to the
flat sides 36,36' and no longer to the narrow sides 26,26' of the leaf 4.
The leaf 4 is moved in the adjusting direction 6 in a manner analogous to
that described for FIGS. 2a-c and FIGS. 3a-c.

[0040]According to FIG. 5, a third linear drive 18 for a leaf 4 again has
two piezoelectric actuators 20,20'. The piezoelectric actuators 20,20'
are both arranged on a narrow side 18 of the leaf 4. Each piezoelectric
actuator 20,20' exerts a frictional force 28,28' with its transducer
24,24' on the narrow side 18. The leaf 4 is again moved in the manner
described for FIGS. 2a-c and FIGS. 3a-c. The arrangement of the
piezoelectric actuators 20,20' on the one hand makes an especially
compact structural design possible; on the other hand, however, the
linear guide 16 must be embodied in such a way as to absorb the
unilaterally acting frictional force 28,28'.

[0041]FIG. 6 is a schematic side view of a radiation therapy device 38
which by means of a retaining device 40 includes a multileaf collimator 2
arranged in a housing. By means of an automatic focusing system not shown
in FIG. 6, the X-ray beam 42 traverses the multileaf collimator 2 in the
beam direction 14 for the purpose of irradiating a tumor 44 of a person
46. By means of its individually displaceable leaves 4 (not shown in the
figure) the multileaf collimator 2 therein establishes the contour 10 for
irradiating the tumor 44, as shown in FIG. 1. Because the person 46 is at
a distance of the order of magnitude of around one meter or more in the
beam direction 14 from the multileaf collimator 2, the X-ray beam 42 will
widen along its path from the multileaf collimator 2 to the tumor 44. In
other words, even slight positioning inaccuracies in the millimeter range
will mean that either diseased tissue within the tumor 44 will not be
covered by the X-ray beam 42 or that healthy tissue surrounding the tumor
44 will be covered by the X-ray beam 42 and damaged by it. Particularly
selective radiation therapy is hence made possible by the improved
positioning of the leaves 4.